1. Field of the Invention
This invention relates to an improvement in the drive circuit for electrostatic deflection type cathode-ray tube oscilloscopes.
2. Discussion of Prior Art
Conventionally, a cascode amplifier has been used as a drive circuit for electrostatic deflection type cathode-ray tube oscilloscopes (noted simply as drive circuit hereafter in this specification) as shown in FIG. 1.
In other words, the drive circuit shown in FIG. 1 is constructed such that input transistors 1 and 2 and output transistors 3 and 4 comprise a push-pull cascode amplifier; the emitters of the input transistors 1 and 2 connect to the power supply -B through feedback resistors 5 and 6 respectively, the bases of the output transistors 3 and 4 are grounded, their collectors connected to the power supply +B through collector load resistors 7 and 8, and the collector outputs of output transistors 3 and 4 are impressed upon the vertical deflection plates of the cathode-ray tube 9.
However, the collector output capacitance 10 of the output transistors 3 and 4, the stray capacitance 11 of the circuit, and the interelectrode static capacitance 12 of the cathode-ray tube are present respectively between the grounds. Consequently, the output impedance and the current of the said drive circuit are determined by the three above-mentioned electrostatic capacitances 10, 11 and 12 not to mention the sensitivity of the cathode-ray tube and the technique employed for leading out the terminals of the deflection plates.
Thus, when operating the above cascode amplifier at high speed and large amplitude, problems arise. For example, when a positive input pulse is impressed upon input terminal 13 and a negative input pulse upon input terminal 14, input transistor 1 and output transistor 3 become conductive and the electric charges of the three electrostatic capacitances 10, 11 and 12 shown at the upper side of FIG. 1 are discharged to the power supply -B through output transistor 3 and input transistor 1 as shown by the dot-and-dash line. On the other hand, input transistor 2 and output transistor 4 become nonconductive and the three electrostatic capacitance 10, 11 and 12 shown at the lower side of FIG. 1 are charged by power supply +B as shown by the broken line in FIG. 1. Consequently, the response speed for output transistor 4 becomes nonconductive is determined by the time constant product of the output circuit of the cascode amplifier--that is, the product of the combined electrostatic capacity C.sub.L of the three electrostatic capacitances 10, 11 and 12 and the resistance value R.sub.L8 of load resistor 8. Thus, a shortcoming is that the response speed is delayed considerably compared to that for output transistor 3 becomes conductive.
In order to achieve a high speed operation, the combined electrostatic capacity C.sub.L and the load resistance R.sub.L must be made sufficiently small. However, when the resistances of load resistors 7 and 8 are reduced, power consumption increases. Also, in order to reduce the electrostatic capacity, the high speed cathode-ray tube is so constructed that the terminals of the deflection plates are led directly out with a side pin to make the lead wire shorter. This makes the high speed cathode-ray tube expensive. Furthermore, the said combined electrostatic capacity C.sub.L has element errors, and in order to correct them, it is necessary to widen the range of correction and adjustment by peaking, etc. So, it has a shortcoming in that special care is required in the selection and control of components used.